[Intel-gfx] [PATCH 0/9] GPU-bound energy efficiency improvements for the intel_pstate driver.

[Peter Zijlstra]

On Thu, Apr 12, 2018 at 11:34:54AM -0700, Francisco Jerez wrote:
> The reason for the energy efficiency problem of iowait boosting is
> precisely the greater oscillation between turbo and idle.  Say that
> iowait boost increases the frequency by a factor alpha relative to the
> optimal frequency f0 (in terms of energy efficiency) required to execute
> some IO-bound workload.  This will cause the CPU to be busy for a
> fraction of the time it was busy originally, approximately t1 = t0 /
> alpha, which indeed divides the overall energy usage by a factor alpha,
> but at the same time multiplies the instantaneous power consumption
> while busy by a factor potentially much greater than alpha, since the
> CPU's power curve is largely non-linear, and in fact approximately
> convex within the frequency range allowed by the policy, so you get an
> average energy usage possibly much greater than the optimal.

Ah, but we don't (yet) have the (normalized) power curves, so we cannot
make that call.

Once we have the various energy/OPP numbers required for EAS we can
compute the optimal. I think such was even mentioned in the thread I
referred earlier.

Until such time; we boost to max for lack of a better option.